The present invention relates generally to the field of power amplifiers, and, more particularly, to an audio power amplifier in which dynamic power output can be increased without increasing continuous power output.
On factor that is particularly important in connection with power amplifiers is power conversion efficiency which measures the ability of an amplifying device to convert the DC power of the bias supply into the AC power delivered to an external load.
For the efficiency of a power amplifier to be very high, the power dissipation on the amplifying device should be kept very low. The power dissipation (PD) on an amplifying device can be defined as follows: EQU PD=(V.sub.cc -V.sub.0) I.sub.0
where
V.sub.cc is the supply voltage PA1 V.sub.0 is the output voltage and PA1 I.sub.0 is the output current.
If the supply voltage is very high as compared to the output voltage, the power dissipation on the device will be high and the efficiency of the amplifier will be low. For maximum efficiency, the supply voltage should be the same or only slightly greater than the output voltage.
Notwithstanding the desirability of maintaining the supply voltage at approximately the level of the output voltage for maximum efficiency, when the amplifier is used in an audio system, it is also important that the supply voltage be sufficiently high to insure that it will always be greater than the output voltage. If the output signal level becomes greater than the supply voltage at any time, output signal clipping will occur resulting in distortion of the amplified signal.
Accordingly, in the usual audio power amplifier system, the supply voltage is maintained at a level sufficient to accommodate the maximum anticipated output signal level. This results in relatively high power dissipation and relatively low amplifier efficiency during periods when the input signal is not at its peak.
In addition, in order to maintain the supply voltage at a sufficiently high level to accommodate all anticipated output signal levels, a power supply with high power output must be utilized to maintain a continuous power output at the necessary level at all times, even though a high power output will usually only be needed during relatively brief periods during the operation of the audio system.